1. Field of the Invention
The present invention relates to a hydraulic pressure control method of a four-speed automatic transmission. More particularly, it relates to a hydraulic pressure control method when an upshift operation changes from a power-on upshift to a power-off upshift.
2. Discussion of Related Art
Automatic transmissions for vehicles selectively actuate a plurality of friction elements such as clutches and brakes, forming part of the automatic transmission, using a hydraulic control system to obtain different gear ratios. FIG. 1 is a representative example of such a hydraulic control system. U.S. Pat. No. 5,003,842 and 5,540,635 to Hatta et al. and Jang, respectively, both hereby incorporated by reference, disclose other examples.
Because the operation of such conventional hydraulic control systems is so well known and readily ascertainable from the above cited patents, a description of that operation will not be repeated for the sake of brevity. During operation a pressure control solenoid valve 2 controls the line pressure in the hydraulic control system in accordance with duty ratios generated by a transmission control unit (TCU) 4. The TCU 4 typically includes a central processing unit, a read only memory, and a random access memory, The TCU 4 controls the operation of the hydraulic control system based on vehicle operating conditions. A plurality of sensors, such as a vehicle speed sensor, engine speed sensor, load or throttle valve sensor, etc, connected to the TCU 4 convey the vehicle operating conditions.
During operation, if a driver depresses the accelerator pedal, placing a large load on the engine, the automatic transmission enters a power-on state. If the accelerator pedal is then released, the automatic transmission enters a power-off state. FIG. 2A illustrates the duty ratio applied to the pressure control solenoid valve 2 by the TCU 4 (see FIG. 1) of the hydraulic control system when the automatic transmission enters the power-on state and then switches to the power-off state. FIG. 2B illustrates the speed of the turbine of the automatic transmission's torque converter when the pressure control solenoid valve 2 operates according to the duty ratio of FIG. 2A.
FIG. 3 illustrates the flow chart of the control method implemented by the TCU 4 during power-on control. In step S2, the TCU 4 outputs duty ratios according to predetermined duty ratios stored therein. As shown in FIG. 2A, at the beginning of the power-on state SS, a minimum duty ratio is output for a first predetermined period of time. Once the first predetermined period of time expires, the duty ratio is set at a first predetermined value, greater than the minimum duty ratio, for a second predetermined period of time. After expiration of the second predetermined period of time, the TCU 4 sets the duty ratio to a second predetermined value, greater than the first predetermined value, for a third predetermined period of time. After the third predetermined period expires, the duty ratio is set to a third predetermined value which is greater than the second predetermined value.
Processing then proceeds to step S4 where the TCU 4 performs conventional duty ratio control such as disclosed in U.S. Pat. No. 5,086,668 to Fujiwara et al., hereby incorporated by reference, using the third predetermined value as an initial value.
Next, in step S6, the TCU 4 determines whether control of the automatic transmission should switch to the power-off state. If not, conventional duty ratio control continues. If control of the automatic transmission switches to the power-off state, then in step S8, the TCU 4 outputs an initial power-off duty ratio D.sub.off to the pressure control solenoid valve 2.
Afterwards, in step S10, the TCU 4 preforms conventional duty ratio control using the initial power-off duty ratio D.sub.off as an initial value.
As a result of switching from the power-on state to the power-off state, a "Run-Up" phenomenon occurs wherein as shown in FIG. 2B the revolutions per minute (hereinafter referred to as a RPM) of the turbine significantly increases and then lowers. The increase is caused by the decrease in the load place on the engine as a result of decreasing the line pressure in the hydraulic control system because the initial power-off duty ratio is much higher then the previously output duty ratio. As shown in FIG. 4, duty ratio is inversely proportional to line pressure.